The present invention relates to control system architecture. More particularly, the present invention relates to an open, interoperable distributed control system in a high performance network environment.
Automatic control systems are critical to all sectors of industry such as process control, discrete control, batch control (process and discrete combined), machine tool control, motion control, and robotics. One of the strongest needs in modern control systems is development and use of xe2x80x9copenxe2x80x9d and xe2x80x9cinteroperablexe2x80x9d systems. Open, interoperable systems allow control devices made by different manufacturers to communicate and work together in the same system without the need for custom programming. xe2x80x9cFieldbusxe2x80x9d is the common term used to describe these types of control systems.
The movement toward open, interoperable fieldbus systems is driven by device manufacturers and end users. Manufacturers want open, interoperable systems because it allows them to sell their products to more end users while reducing development costs. End users want open, interoperable systems so that they can select the best control devices for their system regardless of the device manufacturer.
There has also been a trend toward distribution of control functions into intelligent devices. In centralized control systems, a central controller performs all the control functions.
In distributed control systems, more than one control device operating in the system takes an active role in the control functions. Although both centralized and decentralized systems use a communication network, decentralized systems reduce overall system costs by reducing or eliminating the centralized controller functions between the control devices and the human-machine interface.
In order for distributed control systems to be truly open and interoperable, both the communications system and the user layer (above the communication system layers) must be specified and made open. One of the truly open and interoperable distributed systems is the fieldbus system provided by the Fieldbus Foundation. The FOUNDATION(trademark) fieldbus user layer is described, e.g., in U.S. patent application Ser. No. 08/916,178 (hereafter the xe2x80x9c178xe2x80x9d application) filed Aug. 21, 1997,entitled xe2x80x9cBLOCK-ORIENTED CONTROL SYSTEMxe2x80x9d, and assigned to the assignee of the present application.
The lower speed 31.25 kilobits per second fieldbus (H1) used by the FOUNDATION(trademark) fieldbus is described in part by International Electrotechnical Committee (IEC) Standard IEC 61158, the entirety of which is hereby incorporated by reference herein.
While the FOUNDATION(trademark) fieldbus provides the open and interoperable solution for the H1 control capability, there is a great need to provide an open and interoperable solution for distributed control on a very high performance communication system typically called a fieldbus xe2x80x9cbackbonexe2x80x9d network. The backbone network aggregates information from the lower speed control devices, e.g., the H1 and other control devices, which is used in supervisory and advanced control applications. The backbone is also needed for integration of control information into the enterprise""s Management Information Systems (MIS).
One of the widely accepted standards for high performance communications signaling is Ethernet. Invented by Xerox in the 1970""s, Ethernet has progressed from an initial speed of 10 Megabits per second, to 100 Megabits per second, to 1 Gigabit per second and beyond. Ethernet signaling is specified in an Institute of Electrical and Electronics Engineers (IEEE) standard (IEEE 802.3). Ethernet signaling is the underlying technology used by the Internet. The Internet protocols are specified by the Internet Engineering Task Force (IETF) and are issued as Request For Comment (RFC) specifications.
Although Ethernet/Internet technology provides the basic services for a high performance fieldbus backbone, it does not provide for all of the functions needed for use in distributed control systems. In particular, IEEE and IETF do not have suitable open and interoperable solutions for integration of distributed control systems (e.g., the H1 subsystem), system time synchronization, and fault tolerance.
The method of transferring information from lower speed fieldbuses to the Ethernet used by organizations such as Open DeviceNet(trademark) Vendor Association, Inc., (xe2x80x9cEtherNet/IP,xe2x80x9d) and PROFIBUS International, (xe2x80x9cPROFINetxe2x80x9d) are not suitable for use in the high performance environment because they encapsulate the lower speed protocol packets in an Ethernet frame. This method, known as xe2x80x9ctunneling,xe2x80x9d is common in centralized control systems, but is inadequate for high performance distributed control systems. Although simpler to specify, tunneling would require too many Transport Control Protocol (TCP) connections with the resulting interrupt processing and memory overhead on the devices connected to the fieldbus backbone. In addition tunneling wastes much of the Ethernet bandwidth because the lower speed protocol packets (e.g., the H1 packets) are small and in many cases the Ethernet packet overhead would be bigger than a lower speed protocol packet.
Devices connected to the Ethernet must have a common sense of system time for time stamp and function block scheduling (control) purposes. For high performance distributed control, system time often needs to be accurate to within less than 1 millisecond. Heretofore, there is no known solution that provides this accuracy using the Commercial Off The Shelf (COTS) Ethernet equipment.
Fault tolerance of the Ethernet communication media and devices connected to the Ethernet is required for high performance distributed control applications. There is no known solution that provides the required fault tolerance using standard COTS Ethernet equipment. All of the prior attempts in providing the required fault tolerance require special Ethernet/Internet electronic hardware and/or software, and/or a non-standard xe2x80x9credundancy managerxe2x80x9d device to be added to the Ethernet.
Thus, what is needed is an open, interoperable solution optimized for integration of distributed control systems and other control devices in a high performance fieldbus backbone.
What is also needed is an open, interoperable solution that provides system time synchronization suitable for distributed control applications operable over a high performance fieldbus backbone.
What is also needed is an open, interoperable solution that provides a fault tolerant high performance fieldbus backbone as well as fault tolerant devices that are connected to the fieldbus backbone.
The present invention overcomes the shortcomings described above and provides a new and improved distributed control system, which operates on a high performance backbone, e.g., the standard COTS Ethernet and Internet technology. The embodiments of the present invention are collectively referred to herein as the xe2x80x9cHigh Speed Ethernetxe2x80x9d (HSE). HSE includes the features of the distributed control system described by the ""178 application and FOUNDATION(trademark) fieldbus specifications (which are listed in Appendix A as the Reference Set 1), and further includes three new protocols described in the supporting specifications thereof, which are listed in Appendix A as the Reference Set 2. In particular, the new protocols are referred to herein as: the HSE Field Device Access (FDA) Agent, the HSE System Management Kernel (SMK), and the HSE Local Area Network Redundancy Entity (LRE).
The HSE FDA Agent allows System Management (SM) and Fieldbus Message Specification (FMS) services used by the H1 devices to be conveyed over the Ethernet using standard Internet User Data Protocol (UDP) and Transport Control Protocol (TCP). This allows HSE Devices on the Ethernet to communicate to H1 devices that are connected via a xe2x80x9cHSE Linking Device.xe2x80x9d The HSE FDA Agent is also used by the local Function Block Application Process (FBAP) in a HSE Device or HSE Linking Device. Thus, the HSE FDA Agent enables remote applications to access HSE Devices and/or H1 devices through a common interface.
The HSE SMK ensures that system level functions in each device are coordinated. These functions include system time, addition and removal of devices from the network, and function block scheduling. HSE SMK uses local clock that operates to keep a local time, and keeps the difference between the local time and a system time provided by a time server within a value specified by the time sync class (See Reference Set 1 of Appendix A herein). The local time is used to time stamp events so that event messages from devices may be correlated across the system. Local time is also used to schedule the execution of the local function blocks.
HSE fault tolerance is achieved by operational transparency i.e., the redundancy operations are not visible to the HSE applications. This is necessary because HSE applications are required to coexist with standard MIS applications. The HSE LRE coordinates the redundancy function. Each HSE Device periodically transmits a diagnostic message representing its view of the network to the other HSE Devices on its Ethernet interfaces (commonly called Ethernet xe2x80x9cPortsxe2x80x9d). Each device uses the diagnostic messages to maintain a Network Status Table (NST), which is used for fault detection and Ethernet transmission port selection. There is no central xe2x80x9cRedundancy Managerxe2x80x9d. Instead, each device determines how it should behave in response to faults it detects.